1. Field of Invention
The invention relates to shaping the launch mode trajectory of a cruise missile to more efficiently utilize the booster motor impulse and thereby improve missile launch capability and performance.
2. Description of the Prior Art
The launch mode of a cruise missile from a submerged submarine, surface ship or the ground utilizes a booster rocket motor to propel the missile to a burnout condition that enables the missile to transition to cruise flight. Following the jettison of the spent booster the wings are deployed and the cruise engine started. Stabilized cruise flight begins when the aerodynamic lift is sufficient to support the weight of the missile and reduce any vertical descent rate to zero. The altitude at which this occurs is called the pullout altitude. Aerodynamic lift, for a given missile configuration, is a function of the missile velocity relative to the ambient air. The missile velocity at which the aerodynamic lift is sufficient to support its weight is referred to as v for one "g" (v.sub.1g). Typically, throughout the launch envelope, the booster does not provide the required velocity and missile altitude must be traded for increased velocity as the cruise engine begins to accelerate the missile to the speed necessary to enable pullout prior to crashing to the surface.
Launch performance margin is the excess capability to transition to cruise flight. The measure of performance is pullout altitude. An optimum boost phase trajectory is one that results in the highest pullout altitude. The higher the pullout altitude the greater the launch performance capability. Excess launch performance capability can be used to expand the launch envelope. In the submarine launch case the envelope is defined as a function of launch depth and surface winds. Excess pullout performance can be traded off for deeper launch depths. Surface headwinds increase launch performance capability because they increase the relative velocity. As the headwind increases, eventually a fly away condition exists where the burnout velcoity exceeds v.sub.1g. Tailwinds penalize launch performance in that they reduce the relative velocity resulting in lower pullout altitude. To achieve the highest pullout altitude requires a burnout condition that maximizes the velocity at a positive flight path angle. The burnout altitude is designed to be consistent with maximizing the pullout altitude.
Thus the goal for optimum launch performance for a given booster rocket motor is to maximize the burnout velocity at a positive flight path angle. The trajectory design factors which result in loss of velocity are drag, alignment and gravity. In the submarine launch case, drag losses result from both hydrodynamic and aerodynamic drag. Aerodynamic drag increases with missile angle of attack. Alignment velocity losses occur when the booster thrust vector is not aligned parallel to the velocity vector. Alignment losses are caused by high missile angle of attack and thrust vector control deflections. Far and away the largest velocity losses are produced by gravity. This is the effect of gravity acting opposite to the velocity vector. This term is extremely large for lofted trajectories. The trajectory design problem, however, is that pullout altitude is a sensitive function of flight path angle at burnout as well as burnout velocity. If the flight path angle is too low, there is excessive altitude loss because of the build-up of descent velocity. This problem is further compounded if the burnout altitude is too low. Within the prior art, the design approach is to loft the missile, accepting the large velocity loss due to gravity, in order to achieve burnout at a positive flight path angle. Without lofting the trajectory, the missile flight path angle at burnout becomes too low or even negative which increases the descent rate resulting in lower pullout altitude even though the burnout velocity is higher because of reduced gravity losses.
Booster burnout velocity can only be increased by flying a flatter trajectory at a lower flight path angle. The only way to avoid the small or negative flight path angle at burnout is to provide an upward force on the missile normal to flight path by either inclining the missile thrust vector or by deploying the missile wings during boost to obtain aerodynamic lift. The former approach has the penalty of the large velocity loss due to misalignment of the thrust vector and the velocity vector. The latter approach increases velocity losses slightly due to increased drag caused by a large angle of attack but has significantly reduced velocity losses due to gravity. The net result is a significant improvement in launch performance.
The present invention provides a technique for shaping the launch trajectory by deploying the wings of the cruise missile during the boost phase of the launch enabling a flatter trajectory that significantly increases burnout velocity by reducing gravity losses while maintaining a positive flight path angle. This is accomplished by using wing lift in addition to that provided by the vertical component of the rocket motor. The blend of rocket motor and aerodynamic lift considerably increases the complexity of the missile autopilot. However, greatly increased utilization of the rocket motor impulse is achieved. This means that with the shaped trajectory design a smaller booster rocket motor can be used or, alternatively, the launch performance envelope is vastly improved for the same rocket motor. Although the principal embodiment to be described is for the most complex case, submarine launch mode, the shaped trajectory design is equally applicable to any surface launch mode. This means that with this design a smaller rocket motor can be used to achieve the same launch performance as that of a larger motor using the current design.